Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
Case	Tag   Price	Sales   Tax %	Raw Total   Incl. Sales Tax	Rounded   to Penny	Rounded   to Nickel	Rounded   to Dime
Purchase #1	$76.84	4.170%	$80.0442	$80.04	$80.05	$80.00
Purchase #2	$153.34	11.382%	$170.7932	$170.79	$170.80	$170.80
Purchase #3	$187.88	13.325%	$212.915	$212.92	$212.90	$212.90
Purchase #4	$25.69	8.412%	$27.851	$27.85	$27.85	$27.90
Purchase #5	$248.98	6.801%	$265.9131	$265.91	$265.90	$265.90
Purchase #6	$63.33	12.320%	$71.1323	$71.13	$71.15	$71.10
Purchase #7	$37.76	10.558%	$41.7467	$41.75	$41.75	$41.70
7 purchase totals =	$793.82		$870.3955	$870.39	$870.40	$870.30